By employing the angular spectrum representation, we successfully derive the partial-wave expansion coefficients of the circular Airy beams (CABs) with different polarizations, based on which the scattering of a spherical particle in the CABs is solved exactly. Special attention is focused on exploring the potential applications of the CABs in the abruptly autofocusing (AAF) property and optical manipulation of microparticle by optical force. It is found that both the linear and circular polarizations are the best candidates to implement the AAF property robustly against strong disturbance by a large-sized particle. In addition, although the CABs can be intensively focused to a small region, resulting in an abrupt increase of light intensity by two orders of magnitude at the focal point and enabling three-dimensional stable trapping of a Rayleigh particle near the focal point, the usual CABs fail to generate an optical force outweighing the Brownian force to achieve a stable transverse trapping in the region before the focal point. On the other hand, a Mie particle can be confined transversely in the primary ring and accelerated along the curved trajectory of CABs to the focal point, then pushed further all the way through the focal point rather than being trapped therein, and eventually accelerated further along a straight trajectory in the direction of light propagation over 100 wavelengths, due to the weak diffraction characteristic of a morphed Bessel-like beam of CABs. The exotic curved-straight trajectory transport of particles by the CABs may find applications in the case where the transport path is partly blocked.

We demonstrate that a lateral optical force (LOF) can be induced on chiral nanoparticles immersed in the two interfering plane waves. The LOF can push the chiral nanoparticle sideways, in the direction relying on the helicity of the particle as well as the phase difference between two waves. Analytical theory reveals that the LOF comes mostly from the direct coupling of the optical vorticity with the particle chirality. In particular, the LOF has a magnitude comparable to the gradient force and radiation pressure when the particle chirality is sufficiently large. The LOF may serve for chirality sorting due to its unusual properties and also provide an opportunity for passive chiral spectroscopy.

Lateral optical forces in a direction perpendicular to light propagation have attracted increasing interest in recent years. Up to now, all lateral forces can be attributed to the symmetry breaking in the lateral directions caused by either the morphology of the scatterer geometry or the optical fields impinging on the scatterer. Here we demonstrate, both numerically and analytically, that when an isotropic scatterer breaks the electric-magnetic symmetry, a new type of anomalous lateral force can be induced along the direction of translational invariance where the illumination striking the scatterer has no propagation, field gradient, or spin density vortex (Belinfante’s spin momentum). Our analytical results are rigorous for an arbitrary size scatterer, ensuring the universality of our conclusion. Furthermore, the electric-magnetic symmetry-breaking-induced lateral force is comparable in magnitude to other components of the optical force and reversible in direction for different polarizations of the illuminating light, rendering it capable of practical optical manipulation as well as enriching the understanding of light-matter interaction.

We present the Cartesian multipole expansion theory for computing the optical force acting on a spherical chiral particle immersed in generic monochromatic optical fields. The theory enables us to develop the general formulations for individually calculating the optical gradient and scattering forces (also known as the conservative and nonconservative forces) on a spherical chiral particle of arbitrary size. A set of analytical expressions are then derived for the gradient and scattering forces acting on a chiral particle in arbitrary optical field modeled by a series of homogenous plane waves. As examples of applications, we reveal that, in optical lattice composed of three interferential plane waves, the profiles of the in-plane optical gradient and scattering force acting on a spherical chiral particle show higher degree of symmetry and exhibit invariance with respect to the particle size, material composition, and chirality. The remarkable characteristics are totally masked in the undecomposed optical total force. The rigorous analytical decomposition of optical force sheds some more light on the physical understanding of light-matter interaction, it may also contribute significantly to the design of optical beams for achieving more diversified optical micromanipulation on chiral particles.

We demonstrate numerically and analytically the optical binding between a pair of chiral nanoparticles immersed in a plane wave of linear polarization. The numerical results based on the full wave simulation show that, when the electric field of the incident wave is polarized along the pair axis (the line connecting the particle pair), the closest dimer can be formed at particle separation around $1.25\lambda$, with $\lambda$ denoting the incident wavelength, while when the electric field is polarized normal to the pair axis, the nearest dimer is formed with a separation of one wavelength. In both cases, there exist multiple stable dimer states due to the spatially oscillatory decaying behavior of the binding force with the increase of particle distance. Interestingly, there also appears a lateral optical force exerted on each particle in the direction normal to both the pair axis and light flow, when either or both particles have chirality. Analytical analysis within the dipole approximation indicates that the binding force comes dominantly from the gradient of the optical potential, irrespective of particle chirality. The lateral force, on the other hand, has multiple origins. In addition to the gradient force, the radiation force, the curl force, and the spin force may make significant contributions dependent on the particle chirality, as the particles approach each other.

The working mechanism of optical tweezers is generally believed to be the gradient force arising from the light intensity inhomogeneity of an incident optical field. Here we demonstrate a somewhat counterintuitive phenomenon—optical trapping of chiral particles relying on the gradient force emerging in an optical field with homogeneous light intensity. This type of gradient force is shown to survive even in the Rayleigh limit, where the particle size is far smaller than the illuminating wavelength. By deriving a rigorous analytical expression for the optical force exerted on a chiral particle, the origin of such a gradient force is traced to the gradient of the magnetoelectric energy density, irrespective of the particle size. Besides enriching our understanding of the light-matter interaction, this gradient force is found to have magnitude comparable to the conventional intensity gradient-induced gradient force and to switch its direction for particles with opposite chiralities, rendering it applicable to particle chirality discrimination and identification.